Conduction system dysfunction is a frequent manifestation of heart disease, and in some cases arises from mutations in genes that direct early heart formation. The cardiac conduction system (CCS) consists of specialized cells that function to maintain a normal heartbeat. We hypothesize specific genes and signalling pathways regulate CCS formation and function. Aim 1 is to elucidate genetic differences between and within conduction system cells and working cardiomyocytes. Using our line of mice in which the CCS is delineated by lacZ transgene expression we will isolate the two cell types from both embryo and adult, and generate genetic profiles. Aim 2 focuses on CCS characterization in a mouse mutant with conduction defects similar to Wolff-Parkinson-White syndrome. In this line of mice, signaling through the type I Bone Morphogenetic activin like kinase receptor (ALK3) has been disrupted from early embryogenesis in the atrio-ventricular canal, the site of cushion and node formation. We hypothesize accessory conduction pathways arise in these mice as a result of aberrant CCS development. We will use our conduction system marker line and optical mapping to analyze CCS formation and function in adult and embryonic hearts. [unreadable] [unreadable] [unreadable]